Metropolitan Fire Brigade (MFB) Commander Graeme O'Sullivan told 3AW that he and his wife were on the roof terrace of their hotel in Barcelona when they "heard a loud rumbling sound" followed by "thud sounds".

"We couldn't believe it initially, it was just a little bit too close to home and it was very similar in a lot of ways to the Bourke Street incident."

"That was the vehicle running into two pedestrians and then a short time after that screams," he said.

"It was pretty clear to us it was another one of these incidents -- a deliberate act."

Islamic State has claimed responsibility for the van attack in Barcelona's popular Las Ramblas on Thursday afternoon (local time), an historic street leading to one of the city's central plazas.

Nicolas Carvalho Ochoa via Getty Images
Injured people are tended to near the scene of the terrorist attack in the Las Ramblas area.

O'Sullivan described the chaotic scenes that ensued as resembling the tragic events witnessed in Melbourne earlier this year when Dimitrious Gargasoulas allegedly drove his car through pedestrians, killing six people and injuring 27.

"What we've found out since is that the two offenders driving the van... jumped from the vehicle and started firing shots from the handgun into the crowd and also a restaurant nearby," he said.

"We couldn't believe it initially, it was just a little bit too close to home and it was very similar in a lot of ways to the Bourke Street incident."